This invention relates to ultrasonic examination of surface discontinuities and, more particularly, to an ultrasonic tip diffraction technique for determining the depths of surface cracks.
The problem of determining so-called through-wall crack depths is one of the most difficult and important problems in nondestructive examination (NDE) today. It is particularly important in determining the integrity and remaining lifetime of a component.
Such surface breaking cracks have been sized by various NDE techniques including eddy current, ultrasonic surface waves and electrical potential methods.
Various ultrasonic and non-ultrasonic techniques are known for detecting or measuring flaws. The most common method of sizing is based on a measurement of echo amplitude after the ultrasound is reflected by the surface area of the crack. Examples of prior techniques are found in U.S. Pat. No. 4,679,437, U.S. Pat. No. 4,559,825, U.S. Pat. No. 4,289,033, U.S. Pat. No. 3,924,453, U.S. Pat. No. 3,349,607, U.S. Pat. No. 4,523,468, U.S. Pat. No. 4,467,654 and U.S. Pat. No. 2,725,491. All use angle beam shear waves for ultrasonic examination. U.S. Pat. No. 2,725,491 is primarily directed to an adjustable mounting adapter, as is U.S. Pat. No. 4,117,733. U.S. Pat. No. 4,612,808 describes a probe holder for use on irregular surfaces. U.S. Pat. No. 4,213,183, U.S. Pat. No. 4,052,889 and U.S. Pat. No. 4,393,711 are all based on adaptive learning techniques, which rely upon computer processing and combining of features selected from the ultrasonic signal and transformations of these signals. The system of U.S. Pat. No. 3,981,184 also uses computer processing to correlate ultrasonic indications with the coordinates of their sources and to discriminate between ultrasonic signals of different amplitudes.
U.S. Pat. No. 3,583,211 describes a method to ascertain that ultrasound has reached the entry surface at the correct angle to generate the desired angle beam wave. In U.S. Pat. No. 4,441,369, cracks with rough faceted surfaces are sized by determining locations of all facets, including those at the boundaries of the defect. U.S. Pat. No. 4,274,288 is directed to the use of surface wave interference to determine depth. The surface wave is scattered by the crack. U.S. Pat. No. 4,685,334 detects distributed hydrogen damage by measurement of increased attenuation and does not apply to either detection or sizing of discrete defects. U.S. Pat. No. 4,297,885 does not use ultrasonic methods but employs the detection of elastic energy released as cracks propagate to identify the presence of cracking. U.S. Pat. No. 4,448,080 describes a method for detecting cracks by the change in pressure of a medium pumped into a hole intersecting the crack.
In U.S. Pat. No. 4,475,394, an automated system for correlating particular ultrasonic echoes with specific defects is used. The correlation depends upon comparing probe motion to propagation distance change, keeping in mind the limited beam spread of the transducer. With signal amplitude variation with probe .position known for each defect, the size is determined from the transducer positions giving a preset minimum amplitude.
Ultrasonic tip diffraction is one of the most recently developed techniques for accurately sizing cracks. None of the aforementioned patents employ so-called tip diffracted signals in connection with the various aspects of inspection disclosed in those patents.
Two patents, U.S. Pat. No. 4,570,487 and U.S. Pat. No. 4,522,064, relate to defect sizing using tip diffracted techniques. In both cases, angle beam ultrasonic waves are used. Differences in arrival time of crack root and crack tip signals are used to determine crack size.
The methods previously described for measuring the depths of surface cracks are seriously limited by many physical and geometrical conditions. Restrictions on access creates a need to be able to size cracks in many different types of geometrical configurations that prevent the application of currently available techniques. In addition, many of the current sizing techniques described in the literature are very complex and would be difficult to apply in situations outside the laboratory. In no known case, has a technique been described that utilizes longitudinal waves that are along and parallel to the crack.